1. Field of the Invention
The present invention relates generally to a data processing system including a redundant array of disk drives and more particularly to a method and apparatus for dynamically expanding a redundant array of independent disk drives (RAID) by adding one or more disk drives while the RAID subsystem remains up and running during the expansion of the RAID.
2. Description of the Prior Art
Various types of storage units, such as direct access storage devices (DASDs) are used to store data for known data processing systems. One often used type of DASD is a magnetic disk unit including a number of disks having surfaces with magnetic active material onto which data is written and from which data is read by magnetic read/write heads. In other types of DASDs, optical or other data storage media may be employed.
In a magnetic disk unit, the disks are formatted to define sectors and tracks upon the disk surfaces. Tracks are usually circular regions coaxial with the disk axis where data may be written, and sectors are parts of the tracks capable of storing a predetermined quantity of data written to the disk. Axially aligned tracks on the disks of a DASD are referred to as cylinders. The sectors of a DASD where blocks of data are stored have unique physical data block addresses (DBA). The disks of the DASD spin in unison around a common axis, and the read/white heads, usually one for each surface, are moved radially in unison across the disk surfaces. When data is read from or written to a physical DBA, the heads are moved into alignment with the cylinder containing the track in which the DBA is found, and the data transfer takes place as the sector or sectors of the DBA spin under the head.
Important considerations in the design and use of DASDs are capacity, speed of data transfer and reliability. For reasons including those discussed in Patterson et al., A Case for Redundant Arrays of Inexpensive Disks (RAID)", ACM SIGMOD Conference, Chicago, Ill., Jun. 1-3, 1988, increasing performance levels attained by central processing units (CPUs) and system memory in data processing systems result in the need for larger and faster data storage systems. To achieve these goals of capacity and speed, arrays containing a number of DASDs have been used rather than a single large DASD for data storage.
In a redundant array of independent disk drives (RAID), various modes of operation have been employed to effect reliability. In a RAID type 5 subsystem, a parity based correction is used. The array may include a selected number N+1 of DASDs. Blocks of data called stripes typically ranging from several sectors to a full DASD track are written on N of the DASDs. A parity block is formed by calculating the exclusive-or (XOR) of the N data blocks and storing this parity block on the remaining one of the N+1 DASDs in the array. The parity fields of the array are spread across all N+1 DASDs in the RAID type 5 subsystem. For each set of N blocks of data that are protected by a parity block, the location of the data and parity blocks is varied in a manner that distributes the parity blocks evenly across all DASDs in the RAID type 5 subsystem.
A problem with the RAID type 5 subsystem is the difficulty encountered with the addition of more DASDs to the original N+1 DASD array after the array is in operation and contains useful data. Ideally, the expansion process can be accomplished without disrupting access to the useful data already stored in the array, and with the minimum performance impact to the user of the array. In many known arrangements, the method for adding DASDs to an existing RAID type 5 subsystem involves a save/restore process during which the data on the array is not available to the array user. In other arrangements the addition of one or more DASDs is accomplished without the save/restore process, but with periods during which data in unavailable to the array user as data is being rearranged on the expanded set of DASD devices.